The 3RPS parallel mechanism under unsteady conditions was studied. By establishing geographical coordinate systems and connecting the position of the stabilized platform with the motive vector, operational formulas between normal vectors in upper and rod length were deduced, and the inverse solution derived. The character of velocity and acceleration was analyzed and the trajectory of work space plotted. Analysis indicates the time function of the rod length and the surface of the work space's trajectory are smooth, and no singular position has been found.